1. Field of the Invention
The present invention relates to a portable information terminal having a built-in radio communications device, and more particularly, to a portable information terminal having a built-in radio communications device such as a portable cellular phone.
2. Description of the Background Art
FIG. 3 shows the configuration of a conventional portable information terminal having a built-in radio communications device. The conventional portable information device (a) with a built-in radio communications device [hereinafter referred to simply as an “information device (a)”] comprises a radio communications section (a2), an interface section (a3), an information processing section (a4), and a secondary battery (a8). The radio communications section (a2) has TDMA (time division multiple access)-type radio communications means which transmits and receives data by use of a time division technique. This radio communications section (a2) transmits and receives data by way of an antenna (a1), and the TDMA-type radio communications means comprises a radio circuit (a9) and a radio control circuit (a10). The TDMA-type radio communications means employs a known technique like that employed by a portable digital cellular (PDC) telephone, and hence its explanation is omitted here. The radio control circuit (a10) comprises an unillustrated microprocessor, memory, an I/O control circuit, a serial communications circuit, an audio processing circuit, and a power supply circuit. The radio control circuit (a10) effects wireless control of TDMA processing, communication with an interface, transmission signal processing for converting a voice signal input by way of a microphone (a11) into audio data, and received signal processing for converting the thus-received audio data into an analog audio signal and outputting the thus-converted analog audio signal to a receiver (a12).
The interface (a3) comprises a microprocessor, memory, a serial communications circuit, a parallel communications circuit, and a power supply circuit, which are not illustrated in the drawing. The interface (a3) fulfills a modem function for modulating and demodulating e-mail data or fax data, and a data communications function for transmitting control command data used by the information processing section (a4) for controlling the radio communications section (a2). The modem function comprises encoding and compression of transmission data and decoding and expansion of received data. The e-mail data and the fax data are exchanged between the information processing section (a4) and the radio communications section (a2) by way of a non-audio data bus (a14). Further, the control command data are exchanged between the information processing section (a4) and the radio communications section (a2) by way of a control command bus (a13).
The information processing section (a4) comprises an unillustrated microprocessor, memory, a serial communications circuit, a parallel communications circuit, an I/O circuit, a character input section (a7), and a display section (a6). The character data entered by way of the character input section (a7) are edited into text data and are temporarily stored in memory. When the thus-stored text data are transmitted in the form of an e-mail message, a control command (a call-origination command) is issued for the purpose of bringing the radio communications section (a2) into a communication state. While a communications channel is established after completion of call origination processing, the text data are transmitted to the interface (a3) by way of the non-audio data bus (a14).
The e-mail message received by way of the non-audio data path (a14) is temporarily stored in memory. The thus-stored e-mail message is indicated on the display section (a6) by control operations performed by way of the character input section (a7). The information processing section (a4) serves as a human-machine interface between the operator and the radio communications section (a2). The control information about the radio communications section (a2), which is entered by way of the character input section (a7), is converted into a control command by means of a microprocessor and is transmitted to the interface (a3). As mentioned above, the information processing section (a4) embodies the human-machine interface between the operator and the radio communications section (a2), for example, transmission and receipt operations.
The voltage output from the secondary battery (a8) is supplied to the radio communications section (a2), the interface (a3), and the information processing section (a4), respectively.
The information portable terminal having a built-in radio communications device has conventionally been embodied by means of the above circuit configuration.
In the previously-described circuit configuration, at the time of transmission or reception of an e-mail message, the radio communications section (a2), the interface (a3), and the information processing section (a4) operate simultaneously. As a result, an output impedance of the secondary battery (a8) causes a great drop in output voltage. FIG. 4 shows timings (b1) at which TDMA signals are transmitted and received and voltage fluctuations (b2) occurring at the time of transmission.
Particularly, during a transmission time slot (b21), a significant voltage drop (b22) arises from a load current required by a transmission operation of the radio communications section (a2) and a load current required by operations of the interface (a3) and the information processing section (a4). Because of the voltage drop (b22), the output voltage of the secondary battery (a8) fails to become stable in the vicinity of the final voltage thereof, thus causing an operation failure. Such an operation failure has conventionally been prevented by increasing the operation threshold voltage of the information device (a) to such a voltage as not to induce an operation failure. However, this method also raises a problem of shortening the operation time of the information device (a).
As mentioned above, the radio communications section (a2) has conventionally been operated by means of the information processing section (a4). While the radio communications section (a2) remains in a voice communication state, the information processing section (a4) awaits entry of operation information. To this end, the information processing section (a4) must be held in an operating state. Accordingly, the electric current dissipated by operation in the voice communication state increases, which in turn shortens the operation time.